1. Field of the Invention
The present invention relates to a process for producing a polyoxymethylene. More particularly, it relates to a process for producing a polyoxymethylene having high polymerization degree and high thermal stability by an advantageous industrial process.
Description of the Prior Arts
Polyoxymethylene have excellent mechanical characteristics and have been used for various usages as the engineering plastics. However, a polyoxymethylene produced by polymerizing formaldehyde in the presence of the known catalysts has inferior thermal stability and it could not be used without a modification. Accordingly, the thermal stability of polyoxymethylene has been improved by an esterification or etherification of terminal hydroxyl groups of the polymer. The method of improving thermal stability by modifying the terminal groups requires the post-treatment after the polymerization and accordingly, it is not economical process.
Moreover, the thermal stability in the post-treatment for modifying the terminal groups should be considered and accordingly the thermal stability of the polymer used for the post-treatment should be high.
It has been known to produce a polyoxymethylene by using a catalyst of a reaction product or a coordination complex compound of an organometallic compound having a metal element of the I to III group of the periodic table with a nitrogen-containing compound in Japanese Patent Publication No. 4350/1966. The nitrogen-containing compounds are aliphatic or aromatic acid amides and only acetamide and acetanilide are exemplified.
However, polyoxymethylene having satisfactory thermal stability could not be obtained by using the catalyst of a combination of an organometallic compound and acetamide or acetanilide as a modifier as shown in the results of Preparations No. 18 and 21 in the Examples.